its not "sexual harassment".....its normal harassment technically. BUT it is a simple text message. you were not texting him over and over i assume. But for a single text, there is nothing they can do. They can call the cops and AT MOST you might get a warning. I am not a lawyer, but i clearified it with an Army lawyer buddy of mine.
Personally though, i dont know why you would involve yourself in someone esles sex life. What they do is what they do. They are old enough to make their own mistakes as far as im concerned. Plus i never saw the point of it all.....15 with a 17.... LEGAL.....16 with an 19....ILLEGAL.....WTF....they are both 2 years difference. if he was like 20 or higher.....then i would be like WTF....